Study Notes on Computed Tomography (CT)

COMPUTED TOMOGRAPHY (CT)

PPEA3 Bitesize Revision

• Created by Louise Mckendrick

CONTENTS

• CT Scanner Components

• CT Terminology

• CT Examinations: Dose

• Extravasation in CT

• CT Anatomy

CT SCANNER COMPONENTS

Components Overview

• Gantry

• Couch

• Shielding

• Space for
    - Stretchers
    - Resuscitation team
    - Emergency equipment
    - Medical gasses
    - Injector for IV contrast
    - Control room with consoles and workstations

Detailed Components

Detector

• Detectors receive a beam that exits the patient.

• Technology:
    - Solid state
    - Ultra-fast ceramic scintillator technology

• Efficiency: Absorbs nearly 100% of the photons incident upon them.
    - Electronics convert x-ray energy to electrical signals.
    - Provides digital information to reconstruction computer.

Tube

• Most CT tubes are modified designs based on a standard rotating-anode x-ray tube.

• It produces an x-ray beam that passes through the patient; the densities within the patient will attenuate the beam.

Slip Rings

• Slip rings eliminate long high tension (HT) cables by utilizing electric conductive rings and brushes to transfer electrical power and data.

• Allows for constant rotation of the tube around the patient, giving rise to helical or spiral scanning.

• Eliminates the need for stop-start rotation.

CT TERMINOLOGY

Key Terms

Pitch

• Beam pitch is the distance the table moves in a rotation divided by the active detector width.

• Effects:
    - Higher pitch leads to faster table movement.
    - Increased pitch results in increased noise but lowers the dose.
    - Higher pitch may produce greater artifacts.

Tube Rotation Time

• Represents the time interval needed for a complete 360-degree rotation of the tube detector system around the patient.

CTDI (Computed Tomography Dose Index)

• Definition:
    - A standardized measure of radiation dose output of a CT scanner.

• Measurement Unit: mGy

• Purpose: Allows users to compare radiation output across different CT scanners.

Hounsfield Number

• A quantitative scale for describing radiodensity using an arbitrary unit based on attenuation coefficients.

Window Width

• Refers to the full range of Hounsfield Units (HU) contained in any given CT image, essentially indicating the contrast within an image.

Window Level

• Midpoint of the range of HU of any given CT image.
    - Effect of Decreasing WL: Image becomes brighter, and vice versa.

DLP (Dose Length Product)

• More representative of patient dose, calculated as: $DLP = CTDI_{vol} imes ext{total scan length}$

• Displayed on consoles; recorded by DICOM header.

CT DOSE

Average CT Dose

• Measured in Millisievert (mSv).

• Variability based on scanner types.

• Typical doses include:
    - Non-contrast brain: 2 mSv
    - Contrast-enhanced brain: 4 mSv
    - Routine Chest/Abdomen/Pelvis: 15 mSv
    - Three-phase examination (e.g., Liver, pancreas): 15 mSv
    - Angiography (e.g., mesenteric, peripherals): 15 mSv

CT ANATOMY

Lower Abdomen

1. Aorta

2. Inferior Vena Cava

3. Ureter

4. Left Kidney

5. Small Bowel

6. Appendix

7. Descending Colon

8. Psoas Muscle

9. Erector Spinae Muscle

10. Rectus Abdominus Muscle

11. Spinous Process

Abdomen

1. Liver

2. Spleen

3. Pancreas

4. Gallbladder

5. Right Adrenal Gland

6. Left Adrenal Gland

7. Inferior Vena Cava

8. Aorta

9. Portal Vein

10. Ascending Colon

11. Descending Colon

12. Transverse Colon

13. Stomach (distal)

Chest

1. Trachea

2. Oesophagus

3. Trapezius Muscle

4. Subscapularis Muscle

5. Infraspinatus Muscle

6. Latissimus Dorsi Muscle

7. Erector Spinae Muscle

8. Sternum

9. Ascending Aorta

10. Superior Vena Cava

11. Teres Major Muscle

12. Teres Minor Muscle

NOTES

• Each section provides essential knowledge regarding CT technology, terminology, examination doses, and relevant anatomical structures. Understanding these components is crucial for those engaged in medical imaging and diagnosis using CT scans.